Just saw the system and it's working but ...
I don't like it for one reason: The trees are selectable. There's alot of ways for going around this.
You might know Everwood, the ORPG my team and I are currently working on. If you do know it, you probably also know that we're using a custom made transparent system too. The way I worked around this was a simple model edit, because it's the most efficient method as far as I know. The other way is the GroupEnumUnitsByPlayer (or something) function. I heard it also detects units with the Locust ability, but I haven't tested it to be honest. If you don't know what Locust is, it's an ability which makes your units kinda like a Doodad: It can't be selected or anything.
Well, my suggestion is to test for yourself. I might do it soon because that's the smartest way to make a transparent system if you don't know anything about model editing.